As is well known in the field of modern physics, the study of atomic structure and forces requires the use of extremely short wavelength energy to achieve the resolution necessary to observe subatomic particles. Particle accelerators of various types, such as cyclotrons, synchocyclotrons, and synchotrons, have been used to accelerate hadrons (e.g., protons) into high energy beams which, when collided with a target material, effect the identification and measurement of subatomic particles. In order to detect increasingly smaller particles, the energy to which the hadrons are accelerated must increase because, due to the quantum theory, the wavelength of accelerated particles is inversely proportional to their energy. It is expected that the acceleration of particles to energies of on the order of trillions of electron volts (TeV) will be required to further the state of the art in the study of atomic structure and forces.
Particle accelerators of the synchotron type accelerate particles by way of RF (radio frequency) electrostatic fields through which the beam of particles periodically pass while traveling around a path of substantially constant radius. As is well known, the accelerated particles are maintained within the constant radius path by a transverse magnetic field controlled to increase in magnitude along with the energy to which the particles are accelerated. Some conventional particle accelerators circulate two particle beams, closely parallel to one another but traveling in opposite directions, around the accelerator path. The transverse magnetic field in such accelerators is provided by two separate superconducting magnets, each associated with one of the beam paths.
As the ultimate energy of the acceleration increases, either the radius of the path or the magnitude of the magnetic field (or both) must increase accordingly. Due to the geographical constraints and accompanying construction costs of large radius accelerators, it is desirable to provide extremely high magnetic field magnets for accelerators of the synchotron type, with the fields of up to on the order of several Tesla expected for new accelerators. A discussion of the accelerator cost versus magnetic field strength appears in Perin, "State of the Art in High-Field Superconducting Magnets for Particle Accelerators", Particle Accelerators, Vol. 28 (Gordon and Breach, 1990), pp.147-160.
A conventional design for large field superconducting magnets in particle accelerators is the so-called "cosine-.theta." winding design. According to this design, the superconducting coils surround the magnet bore (within which the beam path will travel) with turns having a density proportional to the cosine of the angle from the horizontal. A high strength non-magnetic laminate collar surrounds the superconducting coil in these designs, with a heavy ferromagnetic shield surrounding both the collar and the coils to provide to reduce field leakage. The Perin article cited hereinabove describes, among others, conventional cosine-.theta. magnets.
The high magnetic fields in such magnets not only maintain the particle beam on the desired path, but also exert inverse Lorentz forces on the magnet structure, particularly the superconducting coils. If a prestressed coil element moves during operation, the energy released can induce local quenching of the superconductivity of the coil element, resulting in localized Joule heating and causing loss of the superconducting state throughout the coil, as described in Huson, et al., "The High Field Superferric Magnet II", Particle Accelerators, Vol. 28 (Gordon and Breach, 1990), pp. 213-218. After a quench event, de-energizing and re-cooling of the coil is therefore necessary in order to regain the superconducting state.
In conventional cosine-.theta. designs, the coils are therefore prestressed by the collar with a pattern of loading forces designed to compensate for the inverse Lorentz forces produced in operation. Such prestress loading has been relatively successful in cosine-.theta. magnets for medium field strengths. However, magnetic fields of on the order of 6.5 to 9.0 Tesla are believed to exceed the limit of the strength of conventional collar materials arranged in the cosine-.theta. design, especially considering that the inverse Lorentz force increases with the square of the magnetic field. It is therefore believed that the practical limit of the cosine-.theta. design will be exceeded for new accelerators of reasonable geographic size.
By way of further background, a superferric accelerator magnet design is described in Colvin, et al., "The High Field Superferric Magnet" Nuclear Instruments and Methods in Physics Research A270 (Elsevier Science Publishers B.V., 1988), pp. 207-211, and in the Huson, et al. cited hereinabove. This superferric magnet is a single bore magnet including a combination of window-frame and cosine-.theta. coils, thus providing a magnet with two modes of operation. Shielding and mechanical prestress is provided by a superferric iron shield and metallic plungers surrounding the bore. While this magnet is contemplated to be useful in high field accelerator applications, its size and weight for a single bore magnet is perceived to be undesirable and costly.
By way of further background, a twin-bore magnet is described in Brianti, "The Large Hadron Collider (LHC) in the LEP Tunnel", Particle Accelerators, Vol. 26 (Gordon and Breach, 1990), pp. 141-150, and in the Perin article cited hereinabove. This magnet includes twin cosine-.theta. magnets surrounded by a single heavy iron shield. It is apparent that the weight and size of this magnet will be quite substantial considering the shielding required, and it is also contemplated that the prestressing of the two cosine-.theta. coils will be relatively complicated.
It is therefore an object of this invention to provide a twin-bore high field magnet which provides a highly uniform magnetic field, particularly where the transverse field through a beam pipe in each bore is substantially independent of radial distance.
It is a further object of this invention to provide such a magnet which includes a flux pipe for return flux so that additional shielding requirements are reduced, if not eliminated.
It is a further object of this invention to provide such a magnet which provides excellent prestressing of the coils and thus reduces the likelihood of localized quenching.
It is a further object of this invention to provide such a magnet that is relatively small and compact.
Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following description together with the drawings.